The present invention relates to a method and apparatus for video signal recording and playback, and in particular to such a method and apparatus whereby a wide-band video signal is divided into a plurality of narrow-band video signals of respective channels, which are respectively recorded by FM recording, and whereby deterioration of the frequency characteristic of a playback signal due to differences between the FM transmission characteristics of the channels is automatically compensated.
The rotary 2-head helical scanning system, typical examples of which are the VHS and the Beta systems, is utilized as the basic operating system for large numbers of video tape recorders (abbreviated in the following to VTR). With such a VTR, electromagnetic heads are attached to the periphery of a rotary drum, spaced apart by 180.degree., whereby 2 fields (1 frame) of a video signal are recorded on two tracks of a magnetic tape during each revolution of the drum. One method of recording a wide-band video signal by utilizing a VTR of this type, i.e. designed for recording a standard TV signal, is to divide the wide-band vide signal into a plurality of narrow-band channels, and to record the divided video signals of these channels simultaneously in parallel. Specifically, assuming that the bandwidth of the wide-band video signal is n times that of the standard signal, the wide-band video signal is divided into n narrow-band video signals of n respective channels, which are recorded simultaneously on respective recording tracks. The following methods are possible for implementing this, in the case of division into two narrow-band video signal channels:
(1) To alternately allocate successive portions of the video signal to the channels each time a sampling point is reached;
(2) To alternately allocate successive horizontal scanning intervals (referred to in the following as 1H intervals) of the video signal to the two channels.
Method (2) above will be described in the following, taking as a practical example a VTR designed for 4 MHz bandwidth NTSC standard operation, with recording of a high-definition television signal employing the MUSE standard (8 MHz bandwidth) being performed by utilizing bandwidth compression. Table 1 shows a comparison between the NTSC and the MUSE standards.
TABLE 1 ______________________________________ Standard Item NTSC MUSE ______________________________________ No. of horizontal 525 1125 scanning lines Horizontal 15.75 33.75 kHz scanning frequency Frame frequency 60 60 Hz ______________________________________
As can be understood from Table 1, each 1H interval of the MUSE standard video signal can be made substantially identical to a 1H interval of an NTSC standard signal, if the MUSE signal is subjected to time-axis expansion by a factor of 2. This is illustrated in FIG. 1. FIG. 1(a) shows horizontal scanning line numbers of a MUSE standard video signal while FIGS. 1(b) and 1(c) respectively show horizontal scanning line numbers of the MUSE signal after time-axis expansion of the horizontal scanning lines of FIG. 1(a) and division into two channels. By applying frequency modulation (FM) to the channel signals obtained in this way, and supplying the resultant FM video signals to respective ones of a pair of electromagnetic heads attached to the periphery of a rotary drum at mutually adjacent positions, one field of the video signal will be recorded simultaneously in parallel on two tracks of a magnetic tape. A second field of the video signal is then recorded, by a second pair of electromagnetic heads which are mounted on the rotary drum periphery at positions spaced apart from the first-mentioned pair by 180.degree..
During playback the original video signal is obtained by performing time-axis compression of the playback signals produced from the channel outputs of the respective heads, and combining the signals thus obtained. However when this is done, due to the fact that horizontal scanning lines which are mutually adjacent within a field of the video signal will be recorded through mutually different channels, deterioration of playback quality will occur due to the fact that there will inevitably be differences between the transmission characteristics of the channels.
A method of eliminating such differences between transmission characteristics which arise in an FM transmission system, i.e. differences in DC level, gain, and non-linearity, has been described in Japanese Patent Laid-open No. 61-46681. With that method, a reference signal such as a ramp signal is inserted into the blanking intervals of the video signal and is recorded together with the video signal. An algorithm is computed for performing compensation of the reference signal contained in the playback signal channels, i.e. after the video signal and reference signal have passed through the FM transmission system. This compensation is applied such as to restore the playback reference signal to a form which is close to that of the original reference signal. After performing compensation of the playback signals in accordance with this algorithm, the playback signals of the respective channels are combined. This method enables differences in DC level, gain and non-linearity between the channels to be eliminated.
However similar differences between the channels will occur in the frequency characteristics of the demodulated signals which are produced from such an FM transmission system, and these differences cannot be eliminated by the prior art method described above. The causes of frequency characteristic deterioration occurring in an FM transmission system are as follows. The generally utilized FM recording method is low-carrier FM. This is due to the fact that in VTR recording, the frequency of the modulation signal is higher, with respect to the FM carrier frequency, than in the case of other types of FM applications. This recording method is made possible by the properties of an FM signal, whereby it is feasible to utilize only one of the sidebands, i.e. the upper or the lower sideband. In addition, as is well known, the VTR recording and playback process results in boosting of low frequency components and attenuation of high frequency components of the recorded signal. Due to this, and since the noise characteristic is flat, if emphasis is applied to the high-frequency range by a playback equalizer and the upper and lower sidebands of the playback signal are of identical amplitude, then the level of noise in the upper sideband range of the playback signal will be relatively high. Thus, the S/N (signal/noise) ratio of the demodulated signal obtained from the playback signal will be poor.
In order to overcome this problem, VTRs generally employ a sloping shape of transmission characteristic. Specifically, the transmission characteristic (i.e. signal amplitude/frequency characteristic) of the overall FM system of a VTR, including the recording system, the heads, the tape, the playback system and the playback equalizer, has the form shown in FIG. 2. In this way, the transmission characteristic of a recording and playback system of a VTR result in emphasis of the low-frequency region and de-emphasis of the high-frequency region, as shown in FIG. 3(a). In FIG. 3(a), B.sub.+1 and B.sub.-1 denote respective rates of change of the upper and lower sidebands. Designating the carrier amplitude as J.sub.O, that of the first upper sideband as J.sub.+1, and that of the first lower sideband as J.sub.-1, the following relationships are true: ##EQU1##
The playback equalizer has a transfer characteristic which is such as to execute correction for the B.sub.-1 characteristic, and thereby produce an overall characteristic of the form shown by the chain line in FIG. 3(a). Such a playback equalizer characteristic is shown in FIG. 3(b). In this way, the amplitude/frequency characteristic of the demodulated FM signal can be made flat, without the need for applying emphasis to the upper sideband. A demodulated signal having a good S/N ratio can thereby be obtained.
With ideal FM transmission of this type, a transmission characteristic of the form shown in FIG. 2 can be initially established, as determined by the recording system, the heads, the tape, the playback system, and the playback equalizer. However in practice, the transmission characteristic of such a system will be different from that described, due to such reasons as differences in the recording and playback characteristics as time elapses, head wear, variations in the frequency characteristics of various types of tape, the temperature characteristics of analog circuits, etc. Furthermore, even initially, the problem of compatibility will arise, i.e. it will be necessary to perform playback of tapes which have been recorded on other units. This is another reason why a specific transmission characteristic cannot be established.
Furthermore with 2-channel recording as described above, horizontal scanning lines of the video signal which appear at mutually adjacent positions on the displayed image will be recorded through the different transmission systems of the respective channels. Differences in the frequency characteristics of these systems therefore result in conspicuous deterioration of image quality.